The present invention relates generally to superconductor containing filaments and processes for preparing them. More particularly, the present invention relates to superconductor containing filaments prepared from multicomponent filaments and processes for preparation of the same.
As used herein, the term xe2x80x9cgreenxe2x80x9d refers to material that has not been through a heat treatment during which organic polymeric materials are combusted.
The terms xe2x80x9cfilamentxe2x80x9d or xe2x80x9cfilamentsxe2x80x9d refer to fibers of long or indefinite length.
The term xe2x80x9cmulticomponent filamentxe2x80x9d refers to filaments having two or more longitudinally continuous distinct domains of constituents.
The terms xe2x80x9cceramicxe2x80x9d or xe2x80x9cceramicsxe2x80x9d refer to inorganic, nonmetallic solids that are subjected to high temperature, generally greater than 600xc2x0 C., during manufacture or use or both.
The terms xe2x80x9csuperconductorxe2x80x9d or xe2x80x9csuperconductorsxe2x80x9d refer to materials that will conduct electricity with no loss of energy due to resistance below a certain critical transition temperature (Tc), below a certain critical current (Jc) and in a magnetic field below a critical strength (Hc). These materials show the Meissner effect (the repulsion of magnetic fields independent of field polarity) below the critical transition temperature (Tc) and critical magnetic field strength (Hc). The critical field strength (Hc) is a function of the temperature. Field strength values are higher at lower temperatures. xe2x80x9cHigh temperature superconductorsxe2x80x9d are those which have a superconducting transition temperature (Tc) above 77K (boiling point of nitrogen under atmospheric pressure).
The term xe2x80x9cviscosexe2x80x9d refers to a viscous solution containing dissolved cellulose xanthate such as is used for spinning into rayon.
The term xe2x80x9crayonxe2x80x9d refers to a semisynthetic fibrous material spun from viscose and composed of regenerated cellulose.
The terms xe2x80x9cprecursorxe2x80x9d or xe2x80x9cprecursorsxe2x80x9d refer to starting or intermediate materials in the fabrication of the superconducting materials that have not yet been processed into a crystalline or semi-crystalline solid capable of superconduction. Metal oxide superconductor precursors are stoichiometric mixtures of nonsuperconducting oxides, nitrates, acetates, carbonates, or other chemical derivatives of potentially superconducting materials that are to be fired or sintered into the superconducting alloy. During firing or sintering, the undesired elements are driven off leaving a superconducting residue.
The discovery of high temperature superconductors opened the pathway for several applications (e.g., supermagnets, generators, electrical energy storage). There are, however, many issues that need to be resolved prior to commercial use of the high temperature superconductors. For example, it is believed that, unlike malleable metals, high temperature superconducting materials cannot be processed by first melting and then forming them to the desired shape, for example, wires. One important field of investigation is, therefore, how to produce superconducting wires having sufficient current density and which are sufficiently insensitive to magnetic fields.
Early work along these lines involved mixing a slurry of a superconducting material into a fiber forming material, and spinning the mixture to form fibers. Fibers made by this process generally lack good fiber properties since high loading of superconducting material into the fiber forming material deteriorates spinning performance. As a result, insufficient superconductor loading resulted in poor superconducting performance, e.g., insufficient consolidation of the superconducting particles. R. B. Cass, xe2x80x9cFabrication of Continuous Ceramic Fiber by the Viscous Suspension Spinning Processxe2x80x9d, Ceramic Bulletin, Vol. 70, No. 3, 1991 describes the loading of viscose with superconducting material which is then spun.
Oxide ceramic superconducting fibers are described in Japanese Kokai Tokkyo Koho Nos. 01,122,511; 01,122,512; and 01,122,521. The fibers may be made by dispersing the superconductor or its source material in an aqueous solution of a water soluble polymer like polyvinyl alcohol; wet spinning the aqueous solution into a solution which precipitates the polymer; and heating the fibers.
Superconducting fibers based on oxide superconductors and products resulting therefrom may be prepared by extruding the superconductor in a binder. Brazilian Patent Application No. 87 03,412 discloses ceramic oxide powder in a polymer binder. The polymer is removed by heating at 100xc2x0 C. and the superconductor is sintered. Japanese Kokai Tokkyo Koho No. 01,176,606 describes a process for making oxide superconducting fiber precursors by dispersing or dissolving an oxide superconductor source material in a solution containing a polymer. The polymer is then spun to form a precursor fiber which is heated. The polymer may be polyvinyl alcohol.
Ceramic superconducting fibers have been spun using nitrate and acetate superconducting precursors in polyacrylic acid gels. Catania, Hovnanian, Cot, xe2x80x9cSuperconducting YBa2Cu3O7xe2x88x92x Fibers From Aqueous Acetate/PAA and Nitrate/PAA Gelsxe2x80x9d, Mat. Res. Bull., Vol. 25, 1990, pp. 1477-1485, describe a lengthwise orientation of the fiber particles. The resulting fibers are described as having poor mechanical properties.
Goto, Sugishita and Kojima, xe2x80x9cA New Fabrication Process of Y1Ba2Cu4O8 Superconducting Filament by Solution Spinning Method Under Ambient Pressurexe2x80x9d, Physica, C 171, 1990, pp. 441-443 describe the preparation of ceramic superconducting fibers by dry spinning superconducting precursors (yttrium, barium and copper acetates) in a polyvinyl alcohol carrier under one atmosphere oxygen pressure. The resulting fiber was considerably porous.
Goto, xe2x80x9cNonaqueous Suspension Spinning of High-TcBaxe2x80x94Yxe2x80x94Cuxe2x80x94O Superconductorxe2x80x9d, Japanese Journal of Applied Physics, Vol. 27, No. 4, April, 1988, pp. L680-L682 discusses the nonaqueous suspension spinning of a superconducting ceramic oxide filament by suspending a fine powder of the oxide precursors in polyvinyl alcohol dimethyl sulfoxide solution containing a dispersant. The suspension is extruded into a precipitating medium of methyl alcohol and coiled on a winding drum. The wound filament is dried and subjected to heat treatment to generate the superconductor.
European Patent Application Publication No. 0 248 432 discloses a process for making a ceramic green body (which may be a fiber) including contacting a superconductor precursor material slurry with a solidifying liquid. The slurry contains a ceramic powder raw material, a binder and a solvent. The binder may be nitrocellulose or cellulose acetate.
It is known to make ceramic fibers from ceramic precursor sheath/core fibers. U.S. Pat. No. 4,863,799 to Mininni et al. describes a preceramic fiber made by melt or solution spinning a sheath/core fiber in which the preceramic material forms the core. Organosilicone preceramic polymers are spun as a core and certain film forming polymers are used for spinning a sheath layer. Cellulose esters of carboxylic acids, such as cellulose acetate, cellulose propionate, cellulose acetate propionate, and the like, may be used as the sheath.
U.S. Pat. No. 4,559,191 to Arons describes another process for preparing a green ceramic fiber using a sheath/core spinning technique. A green ceramic powder is formed into a dispersion or slurry and placed in the core. Suitable sheath forming polymers include cellulosic esters, among others. When the fiber is wet spun, it is extruded into a coagulation bath. The coagulation bath is any nonsolvent for the sheath forming polymer including water, methanol, propanol, ethylene glycol and the like.
It is also known to make bicomponent cellulosic fibers. Viscose rayon bicomponent fibers are described in U.S. Pat. No.2,428,046 to Sisson et al.
There remains a need for a green filament which has superconducting particles at a density and aligned properly to give a usefully high current density and yet still remains manageable, e.g., non-friable.
Also, precursors to superconducting materials typically require special handling techniques to avoid contamination that may hinder superconductivity. Furthermore, most metal oxide superconducting materials are anisotropic with regard to electrical conductivity, i.e., they conduct only in a particular spacial orientation. The particles should all be aligned in this direction to optimize the critical current density. The firing of green wires or filaments containing precursors does not allow for the alignment of the superconducting particles.
One embodiment of the present invention is a process for the production of superconductor containing filaments. The process involves preparing a liquid suspension which contains at least 10 weight percent superconducting material; forming a multicomponent filament having a core of the suspension and a viscose sheath which contains cellulose xanthate; and thereafter, regenerating cellulose from the cellulose xanthate to form a rayon matrix.
In another embodiment, the present invention involves a filament containing superconductor which is a multicomponent filament including one or more embedments of a superconductor material and longitudinally coextensive therewith a rayon matrix surrounding each embedment.
It is an object of the present invention to provide a process for making filaments of superconducting material.
Another object of the present invention is to provide filaments of superconducting material.
Related objects and advantages will be apparent to those of ordinary skill in the art after reading the following detailed description.